It is known in general that, when a transformer is in a state where a residual magnetic flux is present in its iron core, and it is supplied with power to perform no-load excitation, large excitation inrush currents are caused to flow. The excitation inrush currents reach a level several times larger than the rated load current of the transformer. If such large excitation inrush currents flow, the system voltage fluctuates and may affect the consumers when the voltage fluctuation is large.
Accordingly, as a method for suppressing excitation inrush currents of this kind, it is known to use a resistor-equipped breaker in which a closing resistor and a contact are connected in series. The resistor-equipped breaker is connected in parallel with the main contact of a breaker. This resistor-equipped breaker is set closed in advance of the main contact of the breaker. Consequently, the excitation inrush currents are suppressed.
As another suppression method, there is known a method for a three-phase transformer of the directly grounded type, which is supplied with power by three breakers of the single-phase operation type, wherein the method suppresses excitation inrush currents in activating the transformer by first closing an arbitrary one of the breakers of the three phases and then closing the other breakers of the remaining two phases.
Further, there is known a method for a three-phase transformer of the non-effectively grounded type, which is supplied with power by a breaker of the three-phase collective operation type, wherein the method suppresses excitation inrush currents in activating the transformer by measuring the values of residual magnetic fluxes present in the iron core at shutoff of the transformer and then controlling the closing phase position of the breaker.
On the other hand, as a method for converting a three-phase AC (alternate current) voltage to single-phase AC voltages, a Scott connection, Woodbridge connection transformer, and modified Woodbridge connection are known. Transformers of these connection types are used for supplying power to, e.g., single-phase electric furnaces or single-phase AC electric vehicles.
However, the above-described methods for suppressing excitation inrush currents have problems, as follows.
In the case of the excitation inrush current suppression method using a resistor-equipped breaker, since the resistor-equipped breaker needs to be added to an ordinary breaker, the entire breaker system becomes larger.
Further, none of the excitation inrush current suppression methods pays attention to activation of a transformer that converts a three-phase AC voltage to single-phase AC voltages, as mentioned above.
For example, in the case of the method that measures residual magnetic fluxes and then controls the closing phase position of the breaker, the manner of controlling a three-phase transformer for power systems cannot be applied as is to a transformer that converts a three-phase AC voltage to single-phase AC voltages. This is so because transformers of these connection types are structured such that magnetic fluxes present in the transformer iron core cannot be calculated simply by measuring the phase voltages or line voltages on the three-phase AC side.